Bloodied, dirty and stinking, the Ukrainian soldiers who passed through the town of Komsomolske on Saturday morning made for a sorry sight. But they were the lucky ones, who had managed to escape alive from an assault they say involved regular units of the Russian army.

Having fled from encirclement in the town of Ilovaysk, their column of 70 armoured vehicles and hundreds of soldiers was ambushed and shelled, according to one soldier. "Our vehicles were colliding with one another and our tracks were running over our own fighters," said Taras Samchuk, 28, whose 51st brigade was one of the units surrounded.

Many died, some were captured, and about 100 soldiers survived, often "with legs broken or smashed, with shrapnel in our hands, legs, bodies, with smashed teeth and broken noses", he said.

Two weeks ago, Ukraine looked as if it was winning its battle against Russia-backed separatists in eastern Ukraine, but the tide has turned in recent days, with the encirclement at Ilovaysk a key moment. Samchuk, like many Ukrainian fighters, said there is only one reason for this. Instead of fighting a ragtag group of rebels, the Ukrainians have suddenly found themselves fighting the regular Russian army.

Samchuk, who worked as a barman in the western Ukrainian city of Lutsk before being recruited to the army, said he and his comrades rescued a Russian in an armoured vehicle they destroyed near the town. "He told us he served in 8th Chechen brigade [possibly the 8th mountain motor-rifle brigade, based in Chechnya] and he claimed he thought they were sent here for exercises," Samchuk said, adding that the soldier was now being treated in a Kiev hospital.

Nobody knows how many Ukrainians died in Ilovaysk, but the battle for the small town outside Donetsk is likely to go down as one of the defining moments of the conflict. Some were killed in the intense exchanges of shelling in a battle for the town that lasted a week, while others were picked off as they fled, and many more were taken prisoner – more than 500, according to the Ukrainian official in charge of prisoner swaps. In one video released by rebels, a group of captive Ukrainians from volunteer battalions are forced to jump up and down at gunpoint by rebel commanders, reciting derogatory rhymes.

Russia has flatly denied any of its troops are operating in Ukraine, claiming a group of paratroopers captured by Ukrainians deep inside their territory were on a border patrol mission and crossed "accidentally". President Vladimir Putin has repeatedly said that Russia is not party to the "internal Ukrainian conflict". But the evidence is mounting that an impending rebel defeat pushed Moscow to intervene more overtly in the past two weeks. There have been repeated sightings of Russian army ration packaging and of "green men" without insignia similar to those who took part in the annexation of Crimea, as well as satellite images which, Nato claims, show Russian armour inside Ukraine.

A video posted on YouTube this week showed a huge armoured column that appeared to be Russian moving well inside Ukrainian territory. Inside Russia, a regional newspaper has published a long transcript of what it alleges to be recorded conversations with two servicemen of the 76th Pskov airborne division who returned from Ukraine. In their answers, the soldiers relate the story of a company that was almost completely annihilated by Ukrainian artillery. The men say only 10 out of between 80 and 100 from the company survived the shelling.

Lev Schlossberg, a local Pskov politician who obtained the tapes, told the Guardian the soldiers were Russian servicemen but said he could not reveal the identities of the three men or explain how he got hold of the recordings. Schlossberg is recovering from a beating he links to his inquiries about the fate of Pskov soldiers. He said military authorities are coercing relatives of soldiers believed to be in Ukraine into silence, and only a handful are willing to talk.

The Committee of Soldiers' Mothers, a rights organisation, claims up to 15,000 soldiers have been sent over the border in recent weeks. The newspaper Novoya Gazeta published an interview with the mother of 20-year-old sapper Vadim Tumanov, alongside a photograph of an official notice informing the local military commissar of his death.

The last time she heard from her son, he told her he was near the Ukrainian border and "going to war".

The Guardian has found the social network account of a soldier who appears to be fighting in Ukraine. In a post on the Russian Vkontakte network, dated 31 August, Kirill Zdrok from Nizhny Novgorod said he was determined to fight on out of a sense of duty to his fallen comrades. "You know, I will stay here. I cannot sit idly and watch our guys being taken home as Cargo-200s [a Soviet codeword for coffins] … One day you say hello, talk to them, laugh with them, then the next day you find out it's all over – they are no longer alive, heading home in a metal box." He appears frustrated by the secrecy surrounding his mission and his close friends have no idea what is going on.

"You won't see it on TV, hear it on radio or read it in newspapers. You won't find anything on the internet that explains what is really happening where we are now. Honestly, you're better clueless, because the truth is horrible", Zdrok wrote in a post dated 26 August. He had earlier posted a newspaper article about Russian casualties on the border with Ukraine, remarking that they were reconnaissance soldiers from his 9th motor-rifle brigade, based in Nizhny Novgorod.

Semyon Semenchenko, commander of the Ukrainian Donbass volunteer battalion, earlier claimed the 9th brigade was one of the two Russian units that encircled his men at Ilovaysk. The leader of the Donetsk rebels has admitted there are Russian soldiers in the region but says they are not there on orders of the Russian army but "on holiday".

Having routed the Ukrainians at Ilovaysk and taken back other strategic sites such as Luhansk airport and the coastal town of Novoazovsk, regular Russian soldiers have evaporated from view. Days after the surrender of Ilovaysk, the territory between Donetsk and the port of Mariupol is now an eerie no man's land, dotted with burned-out vehicles and the remains of Ukrainian army checkpoints.

The rebels have slowly advanced down the road, though only in small groups. In the town of Telmanove on Tuesday afternoon, a group of seven rebels, dressed in T-shirts and camouflage trousers, were busy using a digger to move concrete blocks into place to form a checkpoint. There was no other sign of the rebels, and certainly no sign of any Russian soldiers, although further up the road two armoured vehicles were spotted driving along dirt tracks, apparently under rebel control.

It is unclear whether the rebels will heed Putin's appeal to stop their advance. In Mariupol, the presumed next rebel target, the Kiev-appointed regional governor, Serhiy Taruta, promised that the city would be defended if the rebels attempted to take it, and said the Russian regular invasion had completely changed the game in eastern Ukraine.

"Without the troops and armour sent over the border, we would have taken back Donetsk by the end of August," he said.